1. Field of the Invention
The present invention relates to the field of electromagnetically controlled drive devices, in particular devices known as "ratio meters".
A particular application for the present invention lies with indicator dials for the dashboards of motor vehicles.
2. Prior Art
It is known that in general a ratio meter comprises:
two crossed control coils; PA1 a radially-magnetized magnet rotatably mounted in an internal chamber defined by said coils; PA1 a shaft associated with the magnet and on its axis, the shaft being adapted to receive the hub of a dial needle; and PA1 magnetic shielding which surrounds the coils, at least in part. PA1 a set of control coils (10, 20); PA1 a frame (50) which carries the set of coils (10, 20); PA1 equipment including a magnet suitable for being pivoted by the set of control coils (10, 20); and PA1 magnetic shielding (100) surrounding the set of coils (10, 20) and the support frame, at least in part, PA1 the device being characterized by the fact that the shielding (100), which is made by stamping a sheet of soft magnetic metal, comprises an annular skirt (110) that is not a circular cylinder.
The control coils are generally carried by a support frame made of plastics material.
The direction in which the needle points depends on the resultant of the magnetic fields created by the two coils.
Thus, if the two coils are fed with signals representing the sine/cosine or the tangent of an input magnitude, then the angle taken up by the magnet relative to a reference position is directly representative of said input magnitude.
Until now, the magnetic shielding has usually been provided in the form of a cylindrical metal bell.
Also until now, the rest position of the moving equipment of such an electromagnetically controlled drive has been set by adding auxiliary magnets or springs, thereby leading to structures that are complex. Document DE-A-3 541 395 describes magnetic shielding formed by a spiral winding of metal foil.
Document GB-A-564 181 describes an indicator having a magnet and a coil and including two screws located inside external shielding and suitable for being adjusted to set the rest position of the device.